Friday, January 06, 2012

Plant microRNAs in Your Blood?

Last month the science magazines and websites were all talking about a paper by Zhang et al. (2012) published in Cell Research. These workers discovered plant micoRNAs in the serum of mice and humans. The microRNAs seem to come from ingested rice. Presumably the micoRNAs are taken up in the intestine and secreted into the blood in small vesicles. The concentration of the major rice miRNAs in serum is about 10 fM or 10×10-15 moles per liter.1

This is one of those findings where the explanation doesn't make a lot of sense but the data seem sound. It seems very unlikely that small plant RNAs could survive the processing and digestion of rice or any other food and even less likely that they would find their way into the bloodstream where they could play a role in regulating mammalian gene expression. I think I'll wait for confirmation.

It's a shame that none of the articles in the popular press expressed any sort of skepticism. That's one of the problems with science journalism. How do you convey the idea that all scientific results are preliminary until they have been confirmed by others?

1. That concentration is far below the concentration where effective binding can occur but the idea seems to be that the micoRNAs are contained in small vesicles that subsequently fuse with liver cells and deliver the rice microRNA to the cytoplasm where it can inhibit translation of specific mammalian RNAs. It's difficult to see how one could get an effective concentration of plant microRNA in one of these mammalian cells.

15 comments
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It seems very unlikely that small plant RNAs could survive the processing and digestion of rice or any other food

To the contrary, this is exactly what we would expect. Our digestive systems let a surprising amount of material into our bodies in undigested or minimally digested forms. For example, we've known since at least the 1960's that intact food proteins and carbohydrates are be found in the circulation after a meal, at concentrations detectable with the immune-based assays of the era. Indeed, the flow of undigested material from the small intestine via the lymph has long been appreciated as the major source of food antigens for those with food allergies.

Granted, RNA may be less stable, but I'm not at all surprised we find it in the blood after a meal.

Where I think this paper has a huge hole is its claims vis-a-vis the effects of the RNA. 10fM is minuscule, and as you mention there isn't a meaningful mechanism to explain how they would enter cells and impact translation. Hell, I spend a lot of money on reagents to get RNA into cells. I only wish it were as easy as an IV injection...

There is probably 10 fM of everything in the blood. Sheesh, that's less than 4x10^9 molecules per whole body consisting of about 7x10^12 cells and containing no more than 7x10^10 hepatocytes. That's less than 0.1 miRNA per liver cell in the best possible scenario. What are these RNAs, totally magic?

The data on RNA stability may have been obtained from a cell-less chemical system, or at least a simple mixture of homogenized cell components, but it seems questionable whether these rudimentary analyses could shed light onto how RNA might behave in complex cell architecture such as plant tissue.

I didn't realize you were talking about in vivo delivery. However: in vivo electroporation is possible and is quite popular in some applications. It was first done in 1989. Tissue(s) can be squeezed between electrodes or needle-like electrodes can be inserted inside. Even blood can be routed through continuous electroporation chamber.

Actually, hydrodynamic tail vein injection of nucleic acids works quite well in mice. However, it requires a huge injection volume and the injected DNA ends up in hepatocytes. IIRC, the injection almost doubles the the bood volume but mice recover within hours from this hypervolumia.

A result like this is not surprising in the least to a cancer researcher like myself. microRNAs are actually quite stable and can survive in the blood for surprisingly long periods of time. In fact, it's a hot area of research to try to look for microRNAs as biomarkers for various tumors. There are even groups doing microRNA microarray profiling of human serum to look for microRNA patters indicative of malignancy.

It is indeed a bit odd that enough could survive uptake and be efficacious. However, I think we're thinking of this as a drug dosage approach, and are ignoring the fact that many people eat rice daily. So, even if we're in the fM range, that's continuous life-long exposure. Is that clinically relevant? Completely unsure!

I work with small RNAs day in and day out. I'm entirely unsurprised at this. The RNA that this paper is referring to is double-stranded which increases stability - most RNases are single-strand specific. Also, they are protected within membrane bound vesicles - so talk of half-life is a moot.

As for low concentration. Again not an issue when it comes to the mechanism by which the human gene is downregulated. There is an internal amplification response in each cell.

(...) we observed that a significant fraction of the circulating RNA appear to originate from exogenous species. With careful analysis of sequence error statistics and other controls, we demonstrated that there is a wide range of RNA from many different organisms, including bacteria and fungi as well as from other species. (...) Some of these RNAs are detected in intracellular complexes and may be able to influence cellular activities under in vitro conditions. These findings raise the possibility that plasma RNAs of exogenous origin may serve as signaling molecules mediating for example the human-microbiome interaction and may affect and/or indicate the state of human health.

(...) Except for miR-168a from the common cereal grains such as corn or rice, the rest of the exogenous miRNAs were from various common household insects, including the housefly, mosquito and bees. One interesting observation is the high variation in the number of reads among individual donors for those insect miRNAs. This was probably caused by the different living conditions and levels of contaminated food consumed by our donors, but this remains to be investigated."

Laurence A. Moran

Larry Moran is a Professor Emeritus in the Department of Biochemistry at the University of Toronto. You can contact him by looking up his email address on the University of Toronto website.

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